The Yiddish expression 'skotsl kumt' is uttered on the occassion of someone's unexpected arrival. Its etymology is unknown. The Biblical Gothic word for 'demon' is 'skohsl'. It is phonetically easy to go from 'skohsl' to 'skotsl'. The Gothic 'h' here designates a preconsonantal 'kh' sound. Such a sound was often lost in the history of Yiddish, compare 'shukh' with derived 'shuster.' The 'sl' cluster can, sometimes become a 'tsl' cluster as in 'pitsl' derived from 'bisl.'
Semantically, the Yiddish and Gothic words are also a good match in that the Yiddish expression can be compared to English 'speak of the Devil and he appears' also said of an unexpected arrival. It can be concluded that Yiddish 'skotsl' is likely descended from Gothic 'skohsl'.